1. Field of the Invention
The present invention relates to a wheel speed detecting device for a vehicle for calculating wheel speeds such as driven wheel speeds and follower wheel speeds which are required, for example, when a slip rate of wheels is determined, in a traction control system or in an antilock brake system.
2. Description of the Related Art
In a vehicle equipped with a traction control system for preventing the excessive slipping of wheels during starting of the vehicle or during rapid acceleration, or for a vehicle equipped with an antilock brake system for preventing the locking of wheels during braking of the vehicle, a slip rate of the wheels is calculated according to the following expression: EQU Slip Rate=[(Driven Wheel Speed-Vehicle Speed)/Vehicle Speed].times.100
When the slip rate of the wheels is calculated according to this expression, the conventional wheel speed detecting devices calculate an average value of left and right driven wheel speeds to supply the same as a driven wheel speed, and calculate an average value of left and right follower wheel speeds to supply the same as a vehicle speed (e.g., see Japanese Patent Application Laid-open No.60331/86). There is also a conventionally known wheel speed detecting device in which a larger one of left and right driven-wheel speeds is selected as a driven wheel speed.
In general, during turning of the vehicle, a difference in speed is generated between the driven and follower wheels because the driven and follower wheels describe different loci. Therefore, to calculate the slip rate of the driven wheels, a vehicle speed corresponding to the travelling locus of the driven wheels should be calculated. In the prior art wheel speed detecting device, however, the vehicle speed is calculated from the speeds of the follower wheels describing the locus different from that of the driven wheels during turning of the vehicle and therefore, a correct vehicle speed corresponding to the driven wheel speeds could not be obtained. Particularly, the loci of the driven and follower wheels are largely different from each other during turning of the vehicle at a low speed and hence, a gap between a vehicle speed corresponding to the driven wheel speeds and a calculated vehicle speed tends to be increased.
An inner and outer wheel speed difference is also generated between the left and right wheel speeds during turning of the vehicle. In this case, for example, in the wheel speed detecting device in which a larger one of the left and right driven wheel speeds is selected as the driven wheel speed, the speed of the driven wheel lying on the outer side during turning of the vehicle is selected as the driven wheel speed. On the other hand, when the output from an engine is excessive during turning of the vehicle to cause slipping of the driven wheels, the driven wheel lying on the inner side during the turning and having a small load tends to be slipped due to the action of a differential. As a result, when the slip rate of the driven wheel lying on the inner side is calculated in order to prevent the excessive slipping of this driven wheel lying on the inner side, the speed of the driven wheel lying on the outer side during the turning is selected and hence, a disagreement is produced between a driven wheel speed required for the calculation and a driven wheel speed actually used for the calculation.